Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced chicken thighs to the skillet and cook for about 5-7 minutes, stirring frequently, until the chicken is browned and cooked through.
Stir in the minced lemongrass, garlic, and ginger. Cook for an additional 2-3 minutes until fragrant.
Add the sliced red bell pepper and snap peas to the skillet. Stir-fry for another 3-4 minutes until the vegetables are tender-crisp.
In a small bowl, mix together the soy sauce, fish sauce, brown sugar, and black pepper. Pour this sauce over the chicken and vegetables, stirring to combine. Cook for another 2 minutes to heat through.
Remove from heat and garnish with fresh cilantro. Serve hot over cooked jasmine rice.